Orthogonal Frequency Division Multiplexing (OFDM) is a well-known multicarrier modulation method that is used in several wireless system standards. Some of the systems using OFDM include 5 GHz high data rate wireless LANs (IEEE802.11a, HiperLan2, MMAC), digital audio and digital video broadcast in Europe (DAB and DVB-T, respectively), and broadband fixed wireless systems such as IEEE802.16a. An QFDM system divides the available bandwidth into very many narrow frequency bands (subcarriers), with data being transmitted in parallel on the subcarriers. Each subcarrier utilizes a different portion of the occupied frequency band.
Spreading can also be applied to the data in an OFDM system to provide various forms of multicarrier spread spectrum. Such spread-OFDM systems are generally referred to as either Spread OFDM (SOFDM), multicarrier CDMA (MC-CDMA), or Orthogonal Frequency Code Division Multiplexing (OFCDM). For systems employing MC-CDMA, spreading is applied in the frequency dimension and multiple signals (users) can occupy the same set of subcarriers by using different spreading codes. For OFCDM, different users are assigned different mutually orthogonal spreading codes, and the spread signals are combined prior to transmission on the downlink. Spreading can be applied in the frequency dimension, or the time dimension, or a combination of time and frequency spreading can be used. In any case, orthogonal codes such as Walsh codes are used for the spreading function, and multiple data symbols can be code multiplexed onto different Walsh codes (i.e., multi-code transmission).
Focusing on OFCDM systems, the orthogonality between Walsh codes is only preserved if the channel is constant over all of the time/frequency resources that are spanned by the Walsh code. This leads to different tradeoffs between time and frequency spreading for different system parameters (e.g., subcarrier and OFDM symbol spacing) and different channel conditions (e.g., delay spread and Doppler spread).
For an OFCDM system with a spreading factor of SF in the time dimension, in which each symbol is represented by SF chips, up to SF Walsh codes can be active on each subcarrier. For channel estimation, one of these Walsh codes can be assigned as a pilot signal (i.e., in the same way that a pilot signal is created in conventional single-carrier CDMA systems such as IS-95). However, a problem with this method is that when time-variations are significant, for example due to vehicular mobility, the orthogonality of the Walsh codes is lost. This causes the pilot channel to suffer interference from the other Walsh codes. Channel estimation is degraded due to this interference. Additionally, when despreading the pilot channel, a single channel estimate results for the entire spread block of SF “chips.” This single channel estimate is not accurate when the channel varies significantly over the block (SF chips). Therefore, a need exists for a method and apparatus for transmission and reception within an OFDM communication system that provides a more accurate channel estimate, and reduces the amount of pilot channel degradation for time-varying channels.